1. Field of the Invention
This invention relates to the field of professional and consumer high-fidelity audio equipment, and more particularly to systems and methods for equalizing audio signals.
2. Description of the Related Art
The audible frequency spectrum (roughly 20 Hz to 20 KHz) can be arbitrarily divided into various ranges. Commonly, sound is classified as low frequency (bass), mid-range frequency, or high frequency (treble). Alternately, the spectrum can be divided into ten octaves (an interval between two frequencies having a two to one ratio), or thirty bands of one-third octave each. A spectral analysis of a signal provided from audio source material characterizes the power of the signal at various frequency ranges. The profile of signal power across these ranges is defined as its spectral balance.
It is well recognized in the audio field that source material, such as music on Compact Disc (CD), has a widely varying spectral balance. This can be explained, in part, by variations in the recording process.
Turning to FIG. 1, a simplified recording process 100 is shown. Within a recording studio 110, microphones typically receive sound waves from the performers and their instruments. The microphone signals are combined in a mixing console and the tracks are mixed down to two stereo channels. The sound is perfected by the production team by listening to the two-channel mix, which is processed by an equalizer and amplifier to drive the monitor speakers. A master mix 130 is made by recording the two-track signal taken before the studio room equalization, and may be further enhanced to correct any deficiencies in the recording process. Master mix 130 is then used to manufacture distribution media 140. Examples include CD 140A, Digital Versatile Disc (DVD) 140B, Laser Disc 140C, VHS 140D, and Cassette 140E. Production team 120 controls various factors throughout recording process 100.
One primary cause of widely varying spectral balance on audio source material is the lack of a standard for the equalization of studio loudspeakers. Other causes include the personal preferences of the production team and tailoring for improved sound quality on car radios and low-cost portable CD players (at the expense of high-end systems). Another unfortunate source is high-frequency hearing loss in members of the production team.
Currently, an end user of an audio system may compensate for variation in spectral balance by using widely available multiband equalizers. Different frequency ranges can be amplified or attenuated by the equalizer to achieve a desired spectral balance. The drawback, however, is that new equalizer settings may be required each time a different source medium is played in order to attain consistent spectral balance. This type of manual setup is not widely employed because it is both time-consuming and often inaccurate. It would therefore be desirable to automate and improve the process of correcting for frequency spectral variations across different audio source media.
The problems outlined above are in large part solved by an audio system in accordance with the present invention. In one embodiment, an audio system is provided that automatically corrects for variations in spectral balance in audio source material. The audio system comprises a power spectrum analyzer and a source correction equalizer unit coupled to receive an audio input signal. The audio system also comprises an index control unit coupled to a spectral balance correction data memory. A reference spectral balance may initially be established by playing a reference source medium with desired sound attributes. The power spectrum analyzer characterizes the spectral balance of the reference source medium, storing the results in non-volatile memory. When a particular source medium is played, the index control unit reads an identifying parameter from the particular source medium. The index control unit then searches the spectral balance correction data memory in order to determine if there is an existing entry for the identifying parameter (and hence, the particular source medium). If the particular source medium has not previously been played, there will be no existing entry. The particular source medium is then played in its entirety, with the index control unit signaling the power spectrum analyzer to characterize the spectral balance of the particular source medium. When the particular source medium is finished playing, an equalizer settings compute unit compares the reference spectral balance and the spectral balance for the particular source medium and calculates equalizer correction settings. These settings, which are also stored to spectral balance correction data memory, may be used to adjust the spectral balance of the particular source medium to match that of the reference source medium. When the particular source medium is subsequently played, the index control unit reads the identifying parameter from the medium, and finds the equalizer correction settings stored in the spectral balance correction data memory. These settings are applied to the source correction equalization unit, which attenuates or amplifies selected frequency ranges of the audio input signal, thereby producing an audio output signal. The audio output signal can also be equalized independently to adjust for listener preferences and room acoustics. In one embodiment, the audio system may also store a plurality of additional reference spectral balances which are user-selectable for various musical styles and listener preferences.
Broadly speaking, the present invention contemplates a method for equalizing an audio input signal provided from an audio source storage medium comprising reading a parameter from the audio source storage medium. The method also comprises using the parameter to index into a memory location which stores particular equalizer settings, and controlling an equalizer unit in accordance with these settings. The equalizer unit is configured to adjust the magnitude of selected frequency ranges of the audio input signal in accordance with the particular equalizer settings, thereby producing an audio output signal.
The present invention also contemplates an audio system configured to equalize an audio input signal provided from an audio source storage medium. The audio system comprises a processing unit coupled to receive the audio input signal, and further coupled to a memory which stores particular equalizer settings. The audio system also comprises an equalizer unit coupled to the processing unit, wherein the equalizer unit is configured to receive the audio input signal. The processing unit is configured to read a parameter from the audio source storage medium, and use this parameter to index into a specific location within the memory which stores the particular equalizer settings. The processing unit is further configured to control the equalizer unit in accordance with the particular equalizer settings, causing the equalizer unit to produce an audio output signal by adjusting the magnitude of selected frequency ranges of the audio input signal.
The present invention further contemplates an audio system configured to equalize an audio input signal provided from an audio source storage medium. The audio system comprises an index control unit configured to read a parameter from the audio source storage medium, and a spectral balance correction memory which stores particular equalizer settings coupled to the index control unit. The audio system further comprises a source correction equalization unit coupled to the spectral balance correction memory bank, wherein the source correction equalization unit is coupled to receive the audio input signal and the particular equalizer settings. The source correction equalization unit is further configured to adjust the magnitude of selected frequency ranges of the audio input signal in accordance with the particular equalizer settings, thereby establishing an audio output signal.